pandas.core.groupby.GroupBy.cumcount

GroupBy.cumcount(ascending=True)

Number each item in each group from 0 to the length of that group - 1.

Essentially this is equivalent to

>>> self.apply(lambda x: Series(np.arange(len(x)), x.index))

Parameters:

ascending : bool, default True

If False, number in reverse, from length of group - 1 to 0.

Examples

>>> df = pd.DataFrame([['a'], ['a'], ['a'], ['b'], ['b'], ['a']],
...                   columns=['A'])
>>> df
   A
0  a
1  a
2  a
3  b
4  b
5  a
>>> df.groupby('A').cumcount()
0    0
1    1
2    2
3    0
4    1
5    3
dtype: int64
>>> df.groupby('A').cumcount(ascending=False)
0    3
1    2
2    1
3    1
4    0
5    0
dtype: int64